Generally, when a specimen is observed on a transmission electron microscope, an image or diffraction pattern of the specimen is projected onto the fluorescent screen, or a specimen image is observed, analyzed, or otherwise processed after such an image or diffraction pattern is recorded on a recording medium such as photographic film. For example, U.S. Pat. No. 4,520,264 discloses an electron microscope which permits the operator to rotate the electron micrograph of a specimen at will so that he or she can most easily observe it. For this purpose, electric currents supplied to the lenses of the imaging lens system are controlled.
In a conventional electron microscope of this construction, the imaging lens system consisting of an objective lens, at least two intermediate lenses, and a projector lens forms the final image either on the fluorescent screen or on a photographic plate. Usually, all of these lenses are of the electromagnetic type and so the final image on the fluorescent screen is rotated through an angle .theta. about the optical axis of the imaging lens system relative to the specimen. This angle .theta. is called the rotational angle and given by ##EQU1## where e is the charge of an electron, m is the rest mass of an electron, Vr is the accelerating voltage acting on the electron beam and corrected for relativistic effects, Bz is the intensity of the magnetic field along the optical axis Z of the imaging lens system, Zs is the position of the specimen on the optical axis, and Ze is the position of the final image on the optical axis. In the above equation, the term (e/8mVr)1/2 has a constant value. The value obtained by integrating Bz from Zs to Ze is equal to the total excitation current fed to the electromagnetic lenses disposed between the specimen and the final image. The total excitation current gives rise to a magnetomotive force represented in ampere-turns (NI).
In the aforementioned electron microscope, the specimen image projected on the fluorescent screen is so rotated that the portion of the image of interest can be most easily observed. In this condition, the image is photographed. Then, a diffraction pattern of the portion of interest is projected onto the fluorescent screen and photographed. During the projection of the diffraction pattern, the lenses are excited in a manner different from when the specimen image is projected. Therefore, the diffraction pattern is projected independent of the rotation of the specimen image projected heretofore. A specimen having a known crystal orientation is used where the crystal orientation is observed or analyzed while comparing the photograph of the specimen image with the previously taken photograph of the diffraction pattern. The difference in orientation between the specimen image and the diffraction pattern is measured in practice. During the observation, the image and the diffraction pattern are made coincident with each other in orientation. This series of operations is very cumbersome to perform and involves inaccuracies.